Hybrid electric vehicles (HEV's) utilize a combination of an internal combustion engine and an electric motor to provide the power needed to propel the vehicle. This arrangement provides improved fuel economy over a vehicle that has only an internal combustion engine. A hybrid vehicle powertrain includes an electric machine, such as an electric motor/generator, that may be used to propel the vehicle either alone or in combination with torque produced by the engine. When operating as a generator, the electric machine generates electric power for use by the vehicle or storage in an associated battery. The engine and motor torques are transferred to vehicle drive wheels through a transmission.
Various powertrain configurations for electric and hybrid electric vehicles have been developed using various types of transmissions and gearing arrangements. One configuration for a hybrid vehicle uses a step-ratio transmission similar to a conventional automatic transmission or automated mechanical transmission and may be referred to as a Modular Hybrid Transmission (MHT) vehicle design. The engine is selectively coupled to the motor by a disconnect clutch with the motor selectively coupled to the transmission gear box by a launch clutch The launch clutch may be integrated with the transmission torque converter in some applications.
Most electric and hybrid vehicles include regenerative braking to improve efficiency and increase the range of the vehicle. During regenerative braking, kinetic energy of the vehicle is converted to electric energy that is used to charge the high voltage battery using the electric machine as a brake and generator. As the vehicle speed decreases, the step-ratio transmission may be downshifted to provide an appropriate gear ratio for the current vehicle speed and requested wheel torque.
Closed loop control of clutch pressure for at least one of the clutches involved in a gear or ratio change may be performed during transmission shifting to provide shift robustness and repeatability. During the ratio change, the associated clutch is “slipping” until the speeds on both sides of the clutch are equal, then the shift is completed. If the shift is proceeding too fast, pressure is removed from the clutch during this slipping phase to prevent a harsh or objectionable shift. Conversely, if the shift is proceeding too slowly, pressure is added to the clutch during this slipping phase. Because the torque transmission through the slipping clutch is proportional to the clutch pressure applied, adding or subtracting clutch pressure to control the speed or completion of the shift will change the output torque delivered to the vehicle wheels, possibly up to 10%. This may be tolerated in conventional vehicles where drivers expect a firmer shift during power-on shifting under positive torque (when accelerating, for example). Similarly, torque variation during shifting while coasting with slightly negative torque (when decelerating, for example) is difficult to detect because the shift is spread over a longer time period. However, in hybrid vehicles, transmission shifting during regenerative braking may become more objectionable using closed-loop clutch pressure control to manage the shift timing and associated shift quality or feel.